Heteroallene Capture and Exchange at Functionalised Heptaphosphane Clusters

Abstract Despite being known for decades the chemical reactivity of homoatomic seven‐atom phosphorus clusters towards small molecules remains largely unexplored. Here, we report that neutral tris(silyl) functionalised heptaphosphane (P7(SiR3)3) cages are capable of heteroallene capture between the P−Si bonds of the cluster. A range of isocyanates and an isothiocyanate were investigated. In the case of isocyanates, silyl bonding at oxygen or nitrogen is regioselectively directed by the functional group on the isocyanate and substituents on the silyl moiety. Above all, we find that captured isothiocyanate molecules can be exchanged for isocyanate molecules, indicative of small molecule catch and release. Small molecule catch and release at these Zintl‐derived clusters reveals their potential application as chemical storage materials or as reusable probes.


Introduction
Zintl clusters, polyatomic clusters of p-block elements, are of continued interest because of their structure, bonding, and physical properties both as solid-state phases and as solutionstate extracted salts. [1] Recent developments in the field are largely focused on the construction and isolation of large and heteroatomic clusters, salt metathesis reactions with Group 14 electrophiles, and coordination chemistry with d-and f-block metals. [1c-k] For example in 2020, Weller and Goicoechea reported the coordination of [Ge 9 ] to catalytically active rhodium to affect the hydrogenation of alkenes. [2] While, Scheschkewitz and co-workers reported the coordination of silicon clusters to catalytically active iridium to mediate the isomerisation of alkenes. [3] In both of these reports, the Group 14 cluster plays the role of a supporting ligand. Early this year, Fässler published the capture of acetonitrile between a bromoborane and functionalised [Ge 9 ] cluster ( Figure 1). [4] This small molecule activation is reminiscent of that observed in frustrated Lewis pair (FLP) chemistry. [5]  The heptaphosphide trianion, [P 7 ] 3À , is an interesting and attractive feedstock because of its structural relationship to red phosphorus. [1h,6] Tris-functionalised [P 7 ] cages are soluble in common laboratory solvents, such as toluene, ether, dichloromethane, and tetrahydrofuran (THF). This solubility allows for rapid assessment of solution-state reactivity. While, red phosphorus is a difficult to study heterogeneous material. [P 7 ] cages can be considered molecular models, and perhaps inform what types of functionalised red phosphorus materials can be targeted for chemical storage.
Examples of small molecule activation with [P 7 ] cages are limited ( Figure 1). In 2012 and 2013, Goicoechea and co-workers employed the protonated heptapnictide cage [HPn 7 ] 2À (Pn = P, As) in hydropnictination reactions with carbodiimides and isocyanates. [7] The same group also found that reactions of [Pn 7 ] 3À (Pn = P, As) with alkynes transferred a [Pn 3 ] À unit to afford 1,2,3-tripnictolides, [8] while reaction with carbon monoxide transferred P À to give the [PCO] À anion. [9] Herein, we find that three equivalents of heteroallene, namely isocyanates (RN=C=O) and an isothiocyanate (RN=C=S), related to the greenhouse gas CO 2 (O=C=O) are captured at these cages. All three of the SiÀ P bonds of silyl functionalised heptaphosphanes (P 7 (SiR 3 ) 3 ) capture a heteroallene via either C=N or C=O bond activation. While small molecule capture into the labile SiÀ P bonds of simple silylphosphines (R 2 PÀ SiR 3 ) has been previously reported, [10] our results represent the first demonstration of this reactivity with Zintl-derived main group clusters. Moreover, not only are heteroallenes captured with this class of compound but, in addition, we find that one heteroallene can be exchanged for another. Small molecule capture followed by unprecedented displacement with Zintlderived clusters is discovered. This type of displacement would be a key advantage when designing re-usable chemical storage materials.

Results and Discussion
First P 7 (SiMe 3 ) 3 (1) was prepared using literature methods, [11] and reacted with three equivalents of phenyl isocyanate in THF overnight (Scheme 1). 29 Si NMR experiments revealed complete consumption of the starting material and formation of a new product with resonances characteristic of SiÀ O and SiÀ N bond formation (see Supporting Information). X-ray diffraction (XRD) studies authenticated the structure of 2. The bridging phosphorus atoms of the cluster are bonded to the isocyanate carbon atoms, while two Me 3 Si units are bonded to the isocyanate nitrogen atoms and one to the oxygen atom. The single-crystal XRD data for 2 showed CÀ O and CÀ N bond lengths of 1.347 (2) Figure 2). The bond lengths are consistent with single bond CÀ O and double bond CÀ N character in the imidate moiety, and vice versa in the amide moiety.
Variable temperature (VT) NMR studies and solid-state (SS) NMR studies were undertaken to probe tautomerization between the imidate and amide moieties of 2. 31 P VT NMR studies between À 70 to 50°C revealed no indication of tautomerization; only 2 was observed. 31 P SS NMR studies gave resonances too broad to be conclusive. The 29 Si SS NMR spectrum was consistent with that collected in the solutionstate and showed no evidence of tautomerization. To investigate if the Me 3 Si-group prefers oxygen bond formation and to force tautomerization the reaction mixture was heated at 50°C, but only decomposition of the cage was observed.
Compound 1 was also treated with one and two equivalents of phenyl isocyanate to isolate the partially inserted products, but only formation of 2 and unreacted starting material could be detected by NMR spectroscopy and XRD studies. This suggests that tris-insertion is preferred over mono-or bisinsertion of the heteroallene.
To investigate the role of the isocyanate on silyl regioselectivity, substituted phenyl isocyanate derivatives with electron-donating and electron-withdrawing groups were reacted with 1. Treatment of 1 with 4-bromophenyl isocyanate afforded the related insertion product 3 (Scheme 2), again with N,N,Osilyl bonding.
In contrast, treatment of 1 with 4-fluorophenyl isocyanate or 4-(trifluoromethyl)phenyl isocyanate showed 1 H and 29 Si NMR spectra consistent with the formation of 4 or 5, respectively, which contains N,O,O-silyl bonding. Reaction of 1 with p-tolyl Scheme 1. Reactions of phenyl isocyanate (R = Ph) with 1.
Formation of insertion products 2-9 could be monitored using 29 Si NMR spectroscopy with the resonance at 7.16 ppm (d, 1 J PSi = 43 Hz) diminishing in intensity and new resonances being observed between 12 and 32 ppm. The silyl regioselectivity upon insertion was determined using 1 H and 29 Si NMR spectroscopy, Figure 5 shows the diagnostic 29 Si NMR signals. In the case of 2, 6-9 silyl regioselectivity was further validated by single-crystal XRD studies. As electron-withdrawing groups are introduced on the isocyanate aromatic ring, upon insertion, the silyl group prefers coordination to the oxygen atom.
To probe the effect of the silyl group on isocyanate insertion compounds 10-12 were prepared where methyl groups on Si Scheme 2. Reactions of isocyanates and an isothiocyanate with 1.   (7), P1À P3 2.1823 (7), P1À P4 2.1994 (7), P2À P5 2.2282 (7), P3À P6 2.2230 (7), P4À P7 2.2183 (7), P5À P6 2.2125 (7), P5À P7 2.1947 (8) (2), C1À O1 1.323(2), O1À Si1 1.7214 (14). Notably, although compounds 1 and 10-12 all showed reactivity towards phenyl isocyanate and tosyl isocyanate, reaction times significantly differed, with 12 requiring nearly 30 days to go to completion. DFT investigations revealed that the fluoride ion affinity (FIA) of the silyl group increases in the order 1 < 10 < 11~12. Consistent with Lewis acidity at the Si centre increasing as methyl groups are replaced with phenyl groups. [P 7 ] bonded silyl groups also showed smaller FIA values when compared to traditional Lewis acids, such as B(C 6 F 5 ) 3 , SbF 5 , and [Me 3 Si] + ( Table 1). Comparison of the 29 Si NMR resonances showed that as methyl groups on the silane are replaced with phenyl groups there is an upfield shift and an increase in the 1 J PSi coupling (Table 1). This is consistent with an increase in P-Si bond strength in the order 1 < 10 < 11 < 12 and the observed increase in reaction times necessary to insert within those bonds. Comparison of the SiÀ P bond distances of 1, 10-12, determined by single-crystal XRD studies, showed no appreciable change and were all within the error of measurement. [12] As methyl groups are replaced with phenyl groups on the silyl unit preferential oxygen bonding is observed. This preference can be attributed to a combination of electronic and steric effects. The increased Lewis acidity (FIA data, Table 1) and steric demand at the silyl group drives toward SiÀ O bond formation.
The 31 P NMR spectra for compounds 9 and 16-18 showed three resonances for the apical, bridging, and basal phosphorus atoms confirming the presence of a single isomer in the solution-state. In contrast, the 31 P NMR spectra for compounds 2-8 and 13-15 revealed multiple isomers in the solution-state and are further discussed in the Supporting Information. Compositional purity for compounds 2-11 and 13-18 was Scheme 3. Reaction of isocyanates with compounds 10-12.   [a] FIAs were computed BP86/SV(p) level of theory, following literature method reported by Greb. [13] further confirmed by elemental analysis and/or mass spectrometry.
To probe reversible binding of the captured heteroallenes exchange reactions were undertaken. It was found that when compound 8 was allowed to react with three equivalents of phenyl isocyanate complete conversion to 2 was achieved after 7 h at 50°C, observed by 31 P NMR spectroscopy (Scheme 4). From this reaction mixture crystals suitable for XRD studies of 2 could also be obtained and are the same as those obtained from its independent synthesis. In a similar fashion, the isothiocyanates captured in 8 could be exchanged for 4fluorophenyl isocyanate, 4-bromophenyl isocyanate, 4-(methoxy)phenyl isocyanate, and p-tolyl isocyanate (Scheme 4). From the latter two reactions crystals of 6 and 7 could be obtained and XRD confirmed the structures. In contrast, treatment of 8 with three equivalents of 4-(trifluoromethyl)phenyl isocyanate under similar conditions showed no reaction. It was also found that the inserted phenyl isocyanates of 2 showed no exchange with phenyl isothiocyanate. Additional efforts to exchange the capture isocyanates of 2, 3, and 5 with another isocyanate required long reaction times at elevated temperatures and led to significant cage decomposition.
DFT investigations were undertaken to understand the thermodynamics of phenyl isothiocyanate displacement in 8 ( Figure 8). By comparing the calculated Gibbs energies It was found that exchange of the captured phenyl isothiocyanate with phenyl isocyanate, 4-(methoxy)phenyl isocyanate, p-tolyl isocyanate, 4-bromophenyl isocyanate, and 4-fluorophenyl isocyanate, is thermodynamically exergonic by À 54.7, À 76.0, À 62.7, À 42.2, and À 38.4 kJ/mol, respectively. Exchange with 4-(trifluoromethyl)phenyl isocyanate is thermodynamically endergonic by 41.6 kJ/mol. These computational findings show that the exchange is more favourable with the introduction of electron-donating groups on the isocyanate, and were consistent with the experimental exchange observations.

Conclusion
In conclusion, we report on the capture of isocyanates and an isothiocyanate between the SiÀ P bonds of tris(silyl) functionalised heptaphosphide clusters. In the case of isocyanates, it was found that both OÀ Si and NÀ Si bond formation was possible. Preferential oxygen bonding was observed when aromatic groups were incorporated on the silyl unit or when isocyanates featuring electron-withdrawing groups were utilised. Further, it was found that in the case of 8, the captured isothiocyanates could be exchanged for a range of isocyanate small molecules. This reactivity is reminiscent of (masked) FLP small molecule activations. [14] Accessing small molecule capture and FLP chemistry with Zintl clusters unlocks their potential application as chemical storage materials or main group catalysts. The heteroallenes studied in this report are valence isoelectronic with CO 2 . Future investigations are focused on CO 2 capture with Zintl clusters and the subsequent reduction chemistry of the activated heteroallenes.

Experimental Section
The general information, experimental procedures, characterisation data, and computational details are available in the Supporting Information.